Ferritic cast iron having spheroidal graphite

ABSTRACT

The invention relates to ferritic cast iron comprising spheroidal graphite, containing, in % by mass,
         from 3.4 to 3.8% of carbon (C)   from 1.5 to 2.1% of silicon (Si)   not more than 0.25% of manganese (Mn)   not more than 0.05% of sulfur (S)   up to 0.6% of usual accompanying elements   balance iron (Fe)       

     In order to achieve a notched impact energy of at least 20 J at a temperature of −10° C. in the case of such a ferritic cast iron comprising spheroidal graphite, the invention proposes that the accompanying elements chromium (Cr), vanadium (V), niobium (Nb) and titanium (Ti) in total be limited to 0.05% by mass. This results in a graphite nodule count of less than or equal to 160/mm 2  in this cast iron. The invention further relates to the use of such a cast iron for cold-sensitive components, in particular in ships or wind power plants.

The invention relates to ferritic cast iron comprising spheroidal graphite, containing, in % by mass:

-   -   from 3.4 to 3.8% of carbon (C)     -   from 1.5 to 2.1% of silicon (Si)     -   not more than 0.25% of manganese (Mn)     -   not more than 0.05% of sulfur (S)     -   not more than 0.6% of usual accompanying elements     -   balance iron (Fe)

The accompanying elements which occur most frequently in ferritic cast iron and usually originate from the starting materials used (scrap and/or pig iron and/or recycled material) are, for example, phosphorus (P), magnesium (Mg), chromium (Cr), nickel (Ni), molybdenum (Mo), copper (Cu), aluminum (Al), titanium (Ti), vanadium (V), niobium (Nb), zirconium (Zr), antimony (Sb), tin (Sn), lead (Pb), cerium (Ce), bismuth (Bi), tellurium (Te) and also further rare earths and the like. Some of these accompanying elements can, particularly if they are carbide formers, adversely affect the microstructure being formed. For this reason, the total content of accompanying elements is generally limited to the value indicated above in the case of commercial ferritic cast iron comprising spheroidal graphite.

The standard DIN-EN 1535 relevant to ferritic cast iron comprising spheroidal graphite leaves the chemical composition in detail and thus also the content of accompanying elements up to the producer, as long as the physical properties sent forth in the standard are adhered to.

Ferritic cast iron comprising spheroidal graphite is basically an iron material which has a microstructure having embedded spheroidal graphite. This particular microstructure results in advantageous properties, including, in particular, a relatively high tensile strength of 320-400 N/mm² at an elongation of from 12 to 22% and also a high notched impact energy of 10 to 12 J at temperatures of from −20° C. to −40° C. (cf., for example, GJS-400-18-LT or GJS-350-22-LT in accordance with the standard DIN-EN-1563:2011). This relatively inexpensive material which can be produced with great process reliability also displays a high deformability before failure, a high resistance to cracks and a high crack arresting capability. For this reason, this material is particularly suitable for applications in which the components are subjected to frequent cyclic collective loads, including at low temperature.

However, there are uses in which the notched impact energy of 12 J at −40° C. which has hitherto been achieved is insufficient. Thus, for example, the rules for the construction of ships which are used in Arctic waters or for the construction of wind power plants which operate in extremely cold locations demand a notched impact energy of at least 20 J at a temperature of −10° C., measured on

Charpy specimens (V-notch) in accordance with EN ISO 148-1:2010. Such a high impact energy has hitherto been able to be achieved only in the case of austenitic cast iron which contains considerable amounts of expensive alloying elements such as nickel.

It is accordingly an object of the invention to provide, in a reliable process, a ferritic cast iron comprising spheroidal graphite of the type mentioned at the outset, which has a notched impact energy of at least 20 J at a temperature of −10° C.

To achieve this object, the invention proceeding from a ferritic cast iron comprising spheroidal graphite of the type mentioned at the outset proposes that the accompanying elements chromium, vanadium, niobium and titanium in total be limited to 0.05% by mass.

It has surprisingly been found in trials carried out in a targeted manner that these four accompanying elements, which are all strong carbide formers, have, alone or in combination, an extraordinarily great influence on the achievable notched impact energy at low temperatures and that, in the case of ferritic cast iron comprising spheroidal graphite of the type mentioned at the outset, the notched impact energy which can be achieved in a reliable process can be increased to more than 20 J at −10° C. when it is ensured that the four abovementioned accompanying elements in the melt are in total limited to 0.05% by mass.

This is advantageously effected by means of a carefully set and monitored charge make-up in order to keep out particular starting materials containing such accompanying elements, or to introduce them in such amounts that the maximum value indicated above is not exceeded.

When these rules are adhered to, a graphite nodule count which is less than or equal to 160 per mm² is established reliably in the ferritic cast iron produced in this way. Such a decrease in the graphite nodule count is known to to an increase in the notched impact energy.

Three specific working examples of the invention will be described in detail below:

EXAMPLE 1

Ferritic cast iron comprising spheroidal graphite and having the following composition:

Carbon (C) 3.600% Silicon (Si) 1.530% Manganese (Mn) 0.198% Sulfur (S) 0.007% Phosphorus (P) 0.010% Magnesium (Mg) 0.052% Nickel (Ni) 0.359% Molybdenum (Mo) 0.001% Copper (Cu) 0.010% Aluminum (Al) 0.014% Zirconium (Zr) 0.005% Antimony (Sb) 0.001% Tin (Sn) 0.001% Lead (Pb) 0.001% Cerium (Ce) 0.001% Bismuth (Bi) 0.001% Tellurium (Te) 0.002% Chromium (Cr) 0.028% Titanium (Ti) 0.003% Vanadium (V) 0.002% Niobium (Nb) 0.001% Balance iron (Fe)

In the case of such a composition, a graphite nodule count of 160/mm² and a notched impact energy of more than 20 J at a temperature of −10° C. are established in the ferritic cast iron produced in this way.

EXAMPLE 2

Ferritic cast iron comprising spheroidal graphite and having the following composition:

Carbon (C) 3.580% Silicon (Si) 1.600% Manganese (Mn) 0.124% Sulfur (S) 0.007% Phosphorus (P) 0.011% Magnesium (Mg) 0.060% Nickel (Ni) 0.208% Molybdenum (Mo) 0.001% Copper (Cu) 0.011% Aluminum (Al) 0.012% Zirconium (Zr) 0.005% Antimony (Sb) 0.001% Tin (Sn) 0.001% Lead (Pb) 0.001% Cerium (Ce) 0.003% Bismuth (Bi) 0.002% Tellurium (Te) 0.002% Chromium (Cr) 0.015% Titanium (Ti) 0.003% Vanadium (V) 0.002% Niobium (Nb) 0.002% Balance iron (Fe)

In the case of such a composition, a graphite nodule count of 130/mm² and a notched impact energy of more than 20 J at a temperature of −10° C. are established in the ferritic cast iron produced in this way,

EXAMPLE 3

Ferritic cast iron comprising spheroidal graphite and having the following composition:

Carbon (C) 3.550% Silicon (Si) 1.600% Manganese (Mn) 0.202% Sulfur (S) 0.006% Phosphorus (P) 0.014% Magnesium (Mg) 0.034% Nickel (Ni) 0.436% Molybdenum (Mo) 0.001% Copper (Cu) 0.004% Aluminum (Al) 0.014% Zirconium (Zr) 0.006% Antimony (Sb) 0.001% Tin (Sn) 0.001% Lead (Pb) 0.001% Cerium (Ce) 0.006% Bismuth (Bi) 0.002% Tellurium (Te) 0.002% Chromium (Cr) 0.011% Titanium (Ti) 0.004% Vanadium (V) 0.002% Niobium (Nb) 0.003% Balance iron (Fe)

In the case of such a composition, a graphite nodule count of 110/mm² and a notched impact energy of more than 20 J at a temperature of −10° C. are established in the ferritic cast iron produced in this way. 

1. A ferritic cast iron comprising spheroidal graphite, containing, in % by mass, from 3.4 to 3.8% of carbon (C) from 1.5 to 2.1% of silicon (Si) not more than 0.25% of manganese (Mn) not more than 0.05% of sulfur (S) not more than 0.6% of usual accompanying elements balance iron (Fe) wherein the accompanying elements chromium (Cr), vanadium (V), niobium (Nb) and titanium (Ti) in total are limited to 0.05% by mass.
 2. The use of ferritic cast iron as claimed in claim 1 for cold-sensitive components.
 3. The use of ferritic cast iron as claimed in claim 2 in ships or wind power plants. 